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Module 2

Data Communication Fundamentals
Version 2 CSE IIT, Kharagpur

Kharagpur
.Lesson 4
Transmission of Digital Signal
Version 2 CSE IIT.

irrespective of the medium. The basis of analog signalling is a constant frequency signal known as a carrier signal. We have seen that analog or digital data traverses through a communication media in the form of a signal from the source to the destination. using different encoding techniques.4.e. 2. either form of data can be encoded into either form of signal. Kharagpur
. the students will be able to: • Explain the need for digital transmission • Explain the basic concepts of Line Coding • Explain the important characteristics of line coding • Distinguish among various line coding techniques o Unipolar o Polar o Bipolar • Distinguish between data rate and modulation rate
2. so that it can traverse a long distance with minimum of attenuation and distortion. Now.1. the data source can be either analog or digital. frequency and phase are being modulated by the source data.4. which is demodulated at the receiving end and the original signal is extracted. the data can be either converted to digital or analog signal. i. What to be used depends on the situation and the available bandwidth. Data can be transmitted using these carrier signals by a process called modulation. There are two alternatives. which is chosen to be compatible with the transmission media being used.
Version 2 CSE IIT. called modulated signal traverses the media. amplitude. The channel bridging the transmitter and the receiver may be a guided transmission medium such as a wire or a wave-guide or it can be an unguided atmospheric or space channel. The resulting signal. where one or more fundamental parameters of the carrier wave. the signal traversing the channel becomes attenuated and distorted with increasing distance.Specific Instructional Objective
On completion.1 Introduction
A computer network is used for communication of data from one station to another station in the network. All the four possibilities are shown in Fig. which is encoded into digital signal. But. Hence a process is adopted to match the properties of the transmitted signal to the channel characteristics so as to efficiently communicate over the transmission media. For digital signalling. Both the approaches have pros and cons.

4.1 Various approaches for conversion of data to signal This lesson will be concerned with various techniques for conversion digital and analog data to digital signal. 2. Depending on the encoding technique used.Data
Digital Analog Analog Digital
Signal
Digital Digital Analog Analog
Approach
Encoding Encoding Modulation Modulation
Figure 2. Figure 2.
2. Kharagpur
. Version 2 CSE IIT. baud rate may be more than or less than the data rate.4.2 Line coding to convert digital data to digital signal No of signal levels: This refers to the number values allowed in a signal. known as line coding.4. known as signal levels.3(b) shows three signal levels to represent binary data.4.2 Line coding characteristics
The first approach converts digital data to digital signal. whereas Fig.
Figure 2. as shown in Fig. Bit rate versus Baud rate: The bit rate represents the number of bits sent per second.3(a) shows two signal levels. to represent data. whereas the baud rate defines the number of signal elements per second in the signal.2. commonly referred to as encoding techniques. Important parameters those characteristics line coding techniques are mentioned below. 2.4.4.

3 (a) Signal with two voltage levels. clock is generated and synchronized from the received signal with the help of a special hardware known as Phase Lock Loop (PLL).4. which is known as the direct-current (DC) component. as shown in Fig. Polar and Bipolar. The DC component also results in unwanted energy loss on the line. This leads to distortion of the signal and may create error at the output. a minimum of one transition per bit interval) in the signal. However.
Version 2 CSE IIT. Kharagpur
. Synchronization: To interpret the received signal correctly.4.4. Usually. the bit interval of the receiver should be exactly same or within certain limit of that of the transmitter. It is necessary to use suitable encoding technique to match with the medium so that the signal suffers minimum attenuation and distortion as it is transmitted through a medium. 2. DC component in a signal is not desirable because the DC component does not pass through some components of a communication system such as a transformer.DC components: After line coding. Cost of Implementation: It is desirable to keep the encoding technique simple enough such that it does not incur high cost of implementation. Any mismatch between the two may lead wrong interpretation of the received signal.4.4. Signal Spectrum: Different encoding of data leads to different spectrum of the signal.3 Line Coding Techniques
Line coding techniques can be broadly divided into three broad categories: Unipolar. (b) Signal with three voltage levels
2. the signal may have zero frequency component in the spectrum of the signal. this can be achieved if the received signal is self-synchronizing having frequent transitions (preferably.3 (a) Signal with two voltage l
(b)
(a)
(b)
Figure 2.
(a) Figure 2.

It is simple but obsolete.5 Unipolar encoding with two voltage levels Polar: Polar encoding technique uses two voltage levels – one positive and the other one negative.4.4. 2. Unfortunately.
Figure 2. Kharagpur
.4.Figure 2.4.6 Encoding Schemes under polar category
Version 2 CSE IIT. In this encoding approach. only two voltage levels are used. Four different encoding schemes shown in Fig. It uses only one polarity of voltage level as shown in Fig.4 Three basic categories of line coding techniques Unipolar: In unipolar encoding technique. DC component present in the encoded signal and there is loss of synchronization for long sequences of 0’s and 1’s.5.4. 2.6 under this category discussed below:
Figure 2. the bit rate same as data rate.

As shown in the figure below. The spectrum of the NRZ-L and NRZ-I signals are shown in Fig.7.8. in NRZ encoding.4. There are two encoding schemes in NRZ: NRZ-L and NRZ-I. Usually a negative voltage is used to represent one binary value and a positive voltage to represent the other. the signal level remains same throughout the bit-period. The main limitations are the presence of a dc component and the lack of synchronization capability.
NRZ – L 1 = low level 0 = high level
NRZ – I • For each 1 in the bit sequence. as shown in Fig. Figure 2. The data is encoded as the presence or absence of a signal transition at the beginning of the bit time. 2.
Version 2 CSE IIT. 2.7 NRZ encoding scheme The advantages of NRZ coding are: • Detecting a transition in presence of noise is more reliable than to compare a value to a threshold. Kharagpur
.4. • NRZ codes are easy to engineer and it makes efficient use of bandwidth. • A transition from one voltage level to the other represents a 1. the receiving side will fail to regenerate the clock and synchronization between the transmitter and receiver clocks will fail. It may be noted that most of the energy is concentrated between 0 and half the bit rate.Non Return to zero (NRZ): The most common and easiest way to transmit digital signals is to use two different voltage levels for the two binary digits.4. When there is long sequence of 0’s or 1’s. the signal level is inverted.

Key characteristics of the RZ coding are: • • • • • Three levels Bit rate is double than that of data rate No dc component Good synchronization Main limitation is the increase in bandwidth
Figure 2. Kharagpur
.4.Return to Zero RZ: To ensure synchronization. 2.9.9 RZ encoding technique
Version 2 CSE IIT. there must be a signal transition in each bit as shown in Fig.4.

the receiver can synchronize properly on that transition. A Manchester code is now very popular and has been specified for the IEEE 802. biphase encoding techniques can be adopted. Kharagpur
.Biphase: To overcome the limitations of NRZ encoding.
Manchester Encoding
Differential Manchester Encoding
Figure 2. 2. as shown in Fig. inversion in the middle of each bit is used for synchronization.4.4.10 Manchester encoding schemes
Version 2 CSE IIT. A binary 1 corresponds to a low-to-high transition and a binary 0 to a high-to-low transition in the middle.
In the standard Manchester coding there is a transition at the middle of each bit period. Manchester and differential Manchester Coding are the two common Biphase techniques in use.3 standard for base band coaxial cables and twisted pair CSMA/CD bus LANs. but due to the predictable transition during each bit time. In Differential Manchester. 2.4. The encoding of a 0 is represented by the presence of a transition both at the beginning and at the middle and 1 is represented by a transition only in the middle of the bit period.11. Key characteristics are: • Two levels • No DC component • Good synchronization • Higher bandwidth due to doubling of bit rate with respect to data rate The bandwidth required for biphase techniques are greater than that of NRZ techniques.10. In Manchester coding the mid-bit
transition serves as a clocking mechanism and also as data. Biphase encoded signals have no DC components as shown in Fig.

General relationship between the two are given below: D = R / b = R / log2L Where. 2. Key characteristics are: • • • • Three levels No DC component Loss of synchronization for long sequences of 0’s Lesser bandwidth
Modulation Rate: Data rate is expressed in bits per second.13 Modulation rate for different encoding techniques Frequency spectrum of different encoding schemes have been compared in Fig.4. L is the number of different signal elements and b is the number of bits per signal element.
Figure 2.4. Modulation rate for different encoding techniques is shown in Fig. 2.14 Frequency spectrum of different encoding schemes
Version 2 CSE IIT. R is the data rate in bps. Kharagpur
. D is the modulation rate in bauds.14. On the other hand.13.Pseudoternary: This encoding scheme is same as AMI. modulation rate is expressed in bauds. but alternating positive and negative pulses occur for binary 0 instead of binary 1.4.4.
Figure 2.

Goals: • • • • • No dc component No long sequences of 0-level line signal No increase in bandwidth Error detection capability Examples: B8ZS.0 + -.
Version 2 CSE IIT. which is used in North America. if the previous pulse was positive. if the previous pulse was negative High Density Bipolar-3 Zeros: Another alternative. A sequence of eight zero’s is replaced by the following encoding A sequence of eight 0’s is replaced by 000+ .+ 0 + -. Kharagpur
.4.Scrambling Schemes: Extension of Bipolar AMI. which is used in Europe and Japan is HDB3. Used in case of long distance applications. The encoded signals are shown in Fig.15. It replaces a sequence of 4 zeros by a code as per the rule given in the following table. 2. HBD3
Bipolar with 8-zero substitution (B8ZS): The limitation of bipolar AMI is overcome in B8ZS. A sequence of eight 0’s is replaced by 000 .

4.1 Pulse Code modulation
Pulse Code Modulation involves the following three basic steps as shown in Fig.Figure 2.4 Analog Data. Kharagpur
.4. 2.4. The device used for conversion of analog data to digital signal and vice versa is called a coder (coder-decoder). video and music can be converted into digital signal communication through transmission media.4.4.15 B8ZS and HDB3 encoding techniques
2. Digital Signals
Analog data such as voice. This allows the use of modern digital transmission and switching equipment’s. There are two basic approaches: Pulse Code Modulation (PCM) Delta Modulation (DM)
2.16: • Sampling – PAM • Quantization • Line coding
Version 2 CSE IIT.

Kharagpur
. With the constraint of a fixed number of levels. during the sampling of voice data.4. The effect of quantization error can be minimized by using a technique known as companding. 8000 samples per second are sufficient for the coding.17. This process introduces an error are known as quantization error.
Version 2 CSE IIT. the situation can be improved using variable step size.16 Basic steps of pulse code modulation Sampling: This process is based on Shannon’s sampling theorem.
Figure 2. instead of using uniform stage sizes. 2. the steps are close together at low signal amplitude and further apart at high signal amplitude as shown in Fig. if n = 4. For example. For example.4. This basic step is known as Pulse Amplitude Modulation (PAM) as shown in Fig.17 Signal outputs after different steps of PCM Quantization: The PAM samples are quantized and approximated to n-bit integer by using analog-to-digital converter.18. Numbers of samples of the signal are taken at regular intervals. This helps to improve the S/N ratio of the signal. In this case. Line coding: The digital data thus obtained can be encoded into one of digital signals discussed earlier. in the frequency range 300 to 4000 Hz. Use of uniform step size leads to poorer S/N ratio for small amplitude signals.4.Figure 2. 2. at a rate higher than twice the highest significant signal frequency. It uses a compressor before encoding and expander after decoding.4. Quantization error depends on step size. then there are 16 (=24) levels available for approximating the PAM signals.

an Digital-to-Analog converter followed by a low-pass filter can be used to get back the analog signal as shown in Fig.4.19. the difference between two consecutive sample values will be small and fewer number of bits can be used with consequent reduction in data rates.4.Figure 2. 2. let us consider voice signal as input with bandwidth of 4 kHz. Based on Nyquist theorem.4. So.19 Conversion of digital to analog signal Limitations: The PCM signal has high bandwidth. Kharagpur
. to send voice signal a data rate of 64 Kbps is required. the Sampling frequency should be 8 kHz. To overcome this problem a technique known as Differential PCM (DPCM) can be used.
Version 2 CSE IIT. Therefore. Since the signal changes slowly. it generates data rate of 64 Kbps. For example. It is based on the observation that voice signal changes slowly.
Figure 2. the difference between two consecutive sample values may be sent.18 The compander At the receiving end. If an 8-bit ADC is used for conversion to digital data.

So. Each sample delta modulation process can be represented by a single binary digit. otherwise a 0 is generated as shown in Fig. Each sample is represented by a single binary digit. 2.Delta Modulation (DM)
Delta Modulation is a very popular alternative of PCM with much reduced complexity. If the difference between analog input and the feedback signal is positive. which moves up or down by one quantization level (a constant amount) at each sampling interval. which makes it more efficient than the PCM technique. then encoded output is 1. Here the analog input is approximated by a staircase function. If the value of the sampled waveform that of the staircase function. a 1 is generated. the analog input is compared to the most recent value of the approximating staircase function at each sampling time. In this modulation technique.4.21. 2.4.4. 2.20 Delta modulation Advantages: Main advantage of Delta Modulation is its simplicity of implementation as shown in Fig. Two important parameters: The size of the step • The sampling rate In the transmitting end.4. The output of the DM is a binary sequence that can be used to reconstruct the staircase function at the receiving end as shown in Fig. we just send the change from previous sample. only one bit is to be sent per sample.
Figure 2.20. Kharagpur
.21.20 shows the Delta modulation operation. instead of sending the entire encoding of each and every sample. which makes it more efficient than the PCM technique. otherwise it is 0. Figure 2. •
Version 2 CSE IIT.4.

2. The steps sizes are large. This problem can be overcome using adaptive delta modulation. Kharagpur
.
Figure 2.Disadvantages: Fixed step size leads to overloading. when the signal changes are large. when the signal changes are small.22.4. but due to its slope as shown in Fig.4.20 Slope overloading
Version 2 CSE IIT. The steps sizes are small.4.21 Implementation of Delta modulation
Figure 2. Overloading occurs not only due to higher voltage.

Between RZ and NRZ encoding techniques. In Differential Manchester. as it requires two signal changes to encode one bit. Distinguish between PAM and PCM signals? Ans: In order to convert Analog data to Digital signal. because B8ZS encoding requires lesser bandwidth than Manchester encoding. Example . initially sampling is done on the analog data by using Sample & Hold (S/H) circuit.Codec (Coder-Decoder) • Digital Data to Analog Signal. Why is it necessary to limit the band of a signal before performing sampling? Ans: It is necessary to limit the bandwidth of a signal before sampling so that the basic requirement of sampling theorem. there is a transition in the middle of each bit period and the receiver can synchronize on that transition. which requires higher bandwidth and why? Ans: RZ encoding requires more bandwidth. In this case. How Manchester encoding helps in achieving better synchronization? Ans: In Manchester encoding. the sampling rate should twice or more than twice the maximum frequency component of the signal. The output of the S/H circuit is known as PAM (Pulse Amplitude Modulated) signal.e. Why do you need encoding of data before sending over a medium? Ans: Suitable encoding of data is required in order to transmit signal with minimum attenuation and optimize the use of transmission media in terms of data rate and error rate. This is known as Nyquist rate. Example .Transmitter • Analog Data to Digital Signal. Example .Review Questions
1. original signal cannot be recovered from the sampled signal. Kharagpur
. 4. If it is violated. and a high-tolow transition represents a 0. Example . Hence better synchronization is achieved. 6. i. What are the four possible encoding techniques? Give examples. a low-to-high transition represents a 1. is satisfied. The PAM signal is then converted to PCM
Version 2 CSE IIT. Ans: The four possible encoding techniques are • Digital Data to Digital Signal. There is a transition at the middle of each bit period. 5. Why B8ZS coding is preferred over Manchester encoding for long distance communication? Ans: The B8ZS encoding is preferred over Manchester encoding.Telephone 3. 2. and a 1 is represented by the absence of a transition at the beginning of a bit period. the midbit transition is only used for synchronization. which serves the purpose of synchronization and encoding of data. 7.Modem • Analog Data to Digital Signal. How does Manchester encoding differ from differential Manchester encoding? Ans: In the Manchester encoding. the encoding of a 0 is represented by the presence of a transition at the beginning of a bit period. 2.

Error due to this quantization is known as quantization error.(Pulse Code Modulated) data by using a Analog-to-Digital (A/D) converter circuit. What is quantization error? How can it be reduced? Ans: To convert analog signal to digital signal. as in case of a voice signal. Kharagpur
. Explain how and in what situation DPCM performs better than PCM Ans: DPCM performs better when the input is slowly changing. This digital data (PCM) is passed through an encoder to generate PCM signal. it is impossible to recover the original signal exactly. In other words.
Version 2 CSE IIT. 9. each sample is approximated by being quantized into some binary codes. 8. Quantization error can be minimized by using non-linear encoding. the analog signal is first sampled and each of these analog samples must be assigned a binary code. As the quantized values are only approximations.